Diffusion barriers for metallic superconducting wires

ABSTRACT

In various embodiments, superconducting wires incorporate diffusion barriers composed of Ta alloys that resist internal diffusion and provide superior mechanical strength to the wires.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/696,330, filed Sep. 6, 2017, which claims the benefit of and priorityto U.S. Provisional Patent Application No. 62/383,676, filed Sep. 6,2016, the entire disclosure of each of which is hereby incorporatedherein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to the formationand processing of superconducting wires that incorporate diffusionbarriers for prevention of low-conductivity phases.

BACKGROUND

A superconducting material exhibits no electrical resistance when cooledbelow its characteristic critical temperature. Although high-temperaturesuperconductor materials, which have critical temperatures higher thanthe 77K boiling point of nitrogen, have been identified, these materialsare often exotic (e.g., perovskite ceramics), difficult to process, andunsuitable for high-field applications. Thus, for practicalsuperconducting applications requiring wires and coils and bundlesthereof, the metallic superconductors Nb—Ti and Nb₃Sn are most oftenutilized. While these materials have critical temperatures below 77K,the relative ease of processing these materials (e.g., drawing intowires), as well as their ability to operate at high currents and highmagnetic fields, have resulted in their widespread use.

Typical metallic superconducting wires feature multiple strands (or“filaments”) of the superconducting phase embedded within a copper (Cu)conductive matrix. While Nb—Ti is sufficiently ductile to be drawn downinto thin wires directly, its applicability is typically limited toapplications featuring magnetic fields having strengths belowapproximately 8 Tesla. Nb₃Sn is a brittle intermetallic phase thatcannot withstand wire-drawing deformation, and thus it is typicallyformed after wire drawing via diffusion heat treatment. Nb₃Snsuperconducting materials may typically be used in applicationsfeaturing magnetic fields having strengths up to at least 20 Tesla.Thus, several different techniques have been utilized to fabricateNb₃Sn-based superconducting wires. For example, in the “bronze process,”a large composite is fabricated from Nb rods and Cu—Sn alloy rods (thatinclude, e.g., 13-15% Sn) surrounding the Nb rods. Since these materialsare ductile, the composite may be drawn down to a suitable diameter, andthen the drawn-down composite is annealed. The heat treatment results ininterdiffusion and the formation of the Nb₃Sn phase at the interfacebetween the Nb and the Cu—Sn. Other processes for forming Nb₃Sn-basedsuperconducting wires similarly involve formation of the brittle Nb₃Snphase after wire drawing. For example, pure Sn or Sn alloys with Cu orMg may be incorporated in the interior of the initial composite andannealed after drawing. Alternatively, Nb filaments may be embeddedwithin a Cu matrix and drawn down into wire. The resulting wire maysubsequently be coated with Sn. The coated wire is heated, forming aSn—Cu phase that eventually reacts with the Nb filaments to form theNb₃Sn phase.

While the techniques detailed above have resulted in the successfulfabrication of metallic superconducting wires utilized for a host ofdifferent applications, the resulting wires often exhibit insufficientelectrical performance. Typical superconducting wires contain many ofthe Nb₃Sn or Nb—Ti filaments described above embedded within, disposedaround, and/or surrounded by a Cu stabilizer that provides the wireswith sufficient ductility for handling and incorporation withinindustrial systems. Although this Cu stabilizer is not itselfsuperconducting, the high electrical conductivity of Cu can enablesatisfactory overall electrical performance of the wire. Unfortunately,various elements from the superconducting filaments (e.g., Sn) may reactwith portions of the Cu stabilizer, forming low-conductivity phases thatnegatively impact the overall conductivity of the entire wire. Whilediffusion barriers have been utilized to shield the Cu stabilizer fromthe superconducting filaments, these barriers tend to have non-uniformcross-sectional areas and may even locally rupture due to non-uniformdeformation during co-processing of the diffusion barrier and the Custabilizer. While such diffusion barriers could simply be made thicker,such solutions impact the overall conductivity of the wire due to thelower electrical conductivity of the diffusion barrier material itself.For example, for cutting-edge and future applications such as newparticle accelerators and colliders, magnets are being designed beyondexisting wire capabilities; such wires will require a non-coppercritical current density of more than 2000 A/mm² at 15 Tesla. As thediffusion barrier is part of the non-copper fraction, minimizing thevolume of any barrier material is important while any strength benefitis advantageous.

In view of the foregoing, there is a need for improved diffusionbarriers for metallic superconducting wires that substantially preventdeleterious reactions involving the Cu stabilizer while remaininguniformly thin so as not to occupy a significant amount of the overallcross-sectional area of the wire.

SUMMARY

In accordance with various embodiments of the present invention, asuperconducting wire and/or precursor thereof (e.g., a compositefilament utilized to form the wire) features a diffusion barrierincluding, consisting essentially of, or consisting of a tantalum (Ta)alloy. The diffusion barrier is typically disposed between at least aportion of the Cu wire matrix and the superconducting filaments, and/orbetween the superconducting filaments and a stabilizing elementincorporated within and/or around the superconducting wire foradditional mechanical strength. In accordance with embodiments of theinvention, monofilaments may each include, consist essentially of, orconsist of a Nb-based core within a Cu-based (e.g., Cu or bronze(Cu—Sn)) matrix, and stacked assemblies of the monofilaments may bedisposed within a Cu-based matrix and drawn down to form compositefilaments. Thus, composite filaments may each include, consistessentially of, or consist of multiple Nb-based monofilaments within aCu-based matrix. A diffusion barrier in accordance with embodiments ofthe invention may be disposed around each composite filament when thecomposite filaments are stacked to form the final wire, and/or adiffusion barrier may be disposed around the stack of compositefilaments and between the stack of composite filaments and an outer Custabilizer or matrix.

In various embodiments, composite filaments are disposed within aCu-based matrix (e.g., a Cu-based tube) and drawn down into thesuperconducting wire (or precursor thereof) and heat treated. One ormore of the composite filaments may themselves incorporate a diffusionbarrier therein, and/or a diffusion barrier may be disposed within theCu-based matrix of the superconducting wire and around the compositefilaments. In various embodiments, the diffusion barrier includes,consists essentially of, or consists of a Ta—W alloy including, e.g.,0.2% to 10% W or 0.2% to 5% W. For example, the diffusion barrier mayinclude, consist essentially of, or consist of an alloy of Ta andapproximately 2.5%-3% W (i.e., Ta-3W). In various embodiments, thediffusion barrier includes, consists essentially of, or consists of aTa—W alloy (e.g., Ta-3W) with one or more additional alloying elementstherein, e.g., alloying elements such as Ru, Pt, Pd, Rh, Os, Ir, Mo, Re,and/or Si. Such alloying elements may be present in the diffusionbarrier individually or collectively at concentrations up to 5% byweight (e.g., between 0.05% and 5%, between 0.1% and 3%, between 0.2%and 2%, between 0.2% and 1%, or between 0.2% and 0.5%). In variousembodiments of the invention, welds formed of Ta—W alloys incorporatingone or more of these additional alloying elements may have grainstructures that are more equiaxed toward the center of such welds; thus,welded tubes formed of these materials for use as diffusion barriers mayexhibit superior mechanical properties and processability when drawndown to small sizes during wire fabrication.

Ta-alloy diffusion barriers in accordance with embodiments of theinvention may also exhibit advantageous ductility due at least in partto low oxygen contents and/or high levels of purity. For example,diffusion barriers in accordance with embodiments of the invention haveoxygen contents less than 500 ppm, less than 200 ppm, less than 100 ppm,or even less than 50 ppm. In addition or alternatively, diffusionbarriers in accordance with embodiments of the invention may havepurities exceeding 99.9%, or even exceeding 99.99%.

Advantageously, Ta-alloy diffusion barriers in accordance withembodiments of the invention have refined grain structures (e.g., smallaverage grain sizes) when compared to conventional diffusion barriermaterials, and this enables the deformation and processing of thediffusion barriers within the superconducting wire to be substantiallyuniform without localized thinning that can rupture the diffusionbarrier and compromise the performance of the wire. The small grain sizeof the diffusion barriers (e.g., less than 20 μm, less than 10 μm, lessthan 5 μm, between 1 and 20 μm, or between 5 and 15 μm) results from thepresence of the alloying element(s), and thus diffusion barriers inaccordance with embodiments of the invention need no additionalprocessing (e.g., forging such as tri-axial forging, heat treatments,etc.) to produce the refined grain structure. Thus, overallmanufacturing costs and complexity may be reduced via use of diffusionbarriers in accordance with the present invention.

The superior grain structure and/or mechanical properties of diffusionbarriers in accordance with embodiments of the present invention enablethe diffusion barriers to provide protection from deleterious diffusionwithin the semiconductor wire without occupying excessive amounts of thecross-sectional (i.e., current-carrying) area of the wire. (In contrast,the use of various other diffusion barriers with lesser mechanicalproperties and/or less refined grain structures would require the use oflarger barriers that would deleteriously impact the ductility,conductivity, and/or various other properties of the final wire.) Wiresin accordance with embodiments of the present invention exhibit littleor no interdiffusion with the Cu matrix while retaining good high-field,high-current superconducting properties below their criticaltemperatures.

The use of Ta-alloy diffusion barriers advantageously enables less ofthe cross-section of the superconducting wire to be occupied by thediffusion barrier, and thus more of the cross-section may be occupied bycurrent-carrying superconducting filaments. However, diffusion-barriermaterials in accordance with embodiments of the invention alsoadvantageously provide additional mechanical strength to thesuperconducting wire while retaining good high-field, high-currentsuperconducting properties below their critical temperatures. In variousembodiments, the mechanical strength of wires may facilitate mechanicaldeformation of the wire (e.g., winding, coiling, etc.) withoutcompromising the electrical performance of the wire and/or withoutcausing cracks or fractures in, or otherwise compromising the mechanicalstability of, the wire and/or its filaments. In various embodiments, thediffusion barrier(s) may collectively occupy at least 2%, at least 3%,at least 4%, or at least 5% of the cross-sectional area of the finalwire. In various embodiments, the diffusion barrier(s) may collectivelyoccupy less than 15%, less than 12%, less than 10%, less than 9%, orless than 8% of the cross-sectional area of the final wire. In thismanner, the diffusion barrier(s) within the superconducting wireprovide, in accordance with various embodiments, the wire with a minimumyield strength (e.g., after any heat treatment of the wire and/or of thefilaments) of at least 75 MPa, or even at least 100 MPa.

The enhanced mechanical strength of superconducting wires in accordancewith embodiments of the invention advantageously enables such wires towithstand Lorentz forces exerted on the wire during operation at highmagnetic field strengths. As known in the art, the “self-field” in amagnet winding is higher than the center-line field and is highest atthe innermost winding. In addition, the current required to create thefield is the same in all the wire in the magnet. The Lorentz force isF=B×I (i.e., magnetic field crossed into current), and the generatedfield is directly proportional to the current I; thus, the force isproportional to the square of the current. For example, the Lorentzforces will be four times higher at 16 Tesla compared to 8 Tesla. Thus,as the applied magnetic field is increased in magnitude, the mechanicalstrength of the wire to withstand the force (perpendicular to bothcurrent and field, via the cross-product relation) must be higher aswell. Superconducting wires in accordance with embodiments of theinvention may be advantageously deployed for applications utilizingmagnetic fields having strengths of at least 2 Tesla, at least 5 Tesla,at least 8 Tesla, or even at least 10 Tesla, i.e., magnetic fluxdensities of at least 20,000 gauss, at least 50,000 gauss, at least80,000 gauss, or even at least 100,000 gauss.

Embodiments of the present invention may also incorporate stabilizingelements within the wires themselves and/or within the compositefilaments utilized to form the wire. For example, embodiments of theinvention may incorporate stabilizing elements that include, consistessentially of, or consist of Ta, a Ta alloy (e.g., an alloy of Ta and Wsuch as Ta-3W), or an alloy of Nb with one or more of Hf, Ti, Zr, Ta, V,Y, Mo, or W, as described in U.S. patent application Ser. No.15/205,804, filed on Jul. 8, 2016 (“the '804 application”), the entiredisclosure of which is incorporated by reference herein.

In an aspect, embodiments of the invention feature a superconductingwire that includes, consists essentially of, or consists of an outerwire matrix, a diffusion barrier disposed within the wire matrix, and aplurality of composite filaments surrounded by the diffusion barrier andseparated from the outer wire matrix by the diffusion barrier. The outerwire matrix includes, consists essentially of, or consists of Cu. Thediffusion barrier includes, consists essentially of, or consists of aTa—W alloy (e.g., a Ta alloy containing 0.2%-10% W). One or more, oreven each, of the composite filaments includes, consists essentially of,or consists of (i) a plurality of monofilaments and (ii) a claddingsurrounding the plurality of monofilaments. The composite-filamentcladding may include, consist essentially of, or consist of Cu. One ormore, or even each, of the monofilaments includes, consists essentiallyof, or consists of a core and a cladding surrounding the core. Themonofilament core may include, consist essentially of, or consist of Nb.The monofilament cladding may include, consist essentially of, orconsist of Cu. The diffusion barrier extends through an axial dimensionof the superconducting wire.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The core of one or more, or even each,of the monofilaments may include, consist essentially of, or consist ofan alloy, pseudo-alloy, or mixture containing Nb and one or more of Ti,Zr, Hf, Ta, Y, or La (e.g., Nb—Ti). The core of one or more, or eveneach, of the monofilaments may include, consist essentially of, orconsist of Nb₃Sn. The diffusion barrier may include, consist essentiallyof, or consist of Ta-3W. The diffusion barrier may additionally containone or more alloying elements selected from the group consisting of Ru,Pt, Pd, Rh, Os, Ir, Mo, Re, or Si. The cross-sectional thickness and/orcross-sectional area of the diffusion barrier may be substantiallyconstant along the thickness of the wire. One or more, or even each, ofthe composite filaments may have a hexagonal cross-sectional shape(i.e., in cross-section perpendicular to the axial dimension of thewire). One or more, or even each, of the monofilaments may have ahexagonal cross-sectional shape (i.e., in cross-section perpendicular tothe axial dimension of the wire).

The wire may include a stabilizing element disposed within the pluralityof composite filaments and surrounded by the diffusion barrier. Thestabilizing element may include, consist essentially of, or consist of aTa alloy containing 0.2%-10% W. At least a portion of the stabilizingelement may be located substantially at the central core of thesuperconducting wire. The stabilizing element may occupy less thanapproximately 20% of a cross-section of the wire, less thanapproximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The stabilizing elementmay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire.

In another aspect, embodiments of the invention feature asuperconducting wire that includes, consists essentially of, or consistsof a wire matrix and a plurality of composite filaments embedded withinthe wire matrix. The wire matrix includes, consists essentially of, orconsists of Cu. One or more, or even each, of the composite filamentsincludes, consists essentially of, or consists of (i) a plurality ofmonofilaments, (ii) a diffusion barrier extending through an axialdimension of the composite filament and surrounding the plurality ofmonofilaments, and (iii) a cladding surrounding the diffusion barrier,the diffusion barrier separating the cladding from the plurality ofmonofilaments. The composite-filament diffusion barrier includes,consists essentially of, or consists of a Ta—W alloy (e.g., a Ta alloycontaining 0.2%-10% W). The composite-filament cladding includes,consists essentially of, or consists of Cu. One or more, or even each,of the monofilaments includes, consists essentially of, or consists of acore and a cladding surrounding the core. The monofilament core mayinclude, consist essentially of, or consist of Nb. The monofilamentcladding may include, consist essentially of, or consist of Cu.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The core of one or more, or even each,of the monofilaments may include, consist essentially of, or consist ofan alloy, pseudo-alloy, or mixture containing Nb and one or more of Ti,Zr, Hf, Ta, Y, or La (e.g., Nb—Ti). The core of one or more, or eveneach, of the monofilaments may include, consist essentially of, orconsist of Nb₃Sn. The diffusion barrier may include, consist essentiallyof, or consist of Ta-3W. The diffusion barrier may additionally containone or more alloying elements selected from the group consisting of Ru,Pt, Pd, Rh, Os, Ir, Mo, Re, or Si. The cross-sectional thickness and/orcross-sectional area of the diffusion barrier may be substantiallyconstant along the thickness of the wire. One or more, or even each, ofthe composite filaments may have a hexagonal cross-sectional shape(i.e., in cross-section perpendicular to the axial dimension of thewire). One or more, or even each, of the monofilaments may have ahexagonal cross-sectional shape (i.e., in cross-section perpendicular tothe axial dimension of the wire).

The wire may include a stabilizing element disposed within the pluralityof composite filaments. The stabilizing element may include, consistessentially of, or consist of a Ta alloy containing 0.2%-10% W. At leasta portion of the stabilizing element may be located substantially at thecentral core of the superconducting wire. The stabilizing element mayoccupy less than approximately 20% of a cross-section of the wire, lessthan approximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The stabilizing elementmay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire.

In yet another aspect, embodiments of the invention feature asuperconducting wire that includes, consists essentially of, or consistsof an inner wire stabilizing matrix, a diffusion barrier disposed aroundthe wire stabilizing matrix, and a plurality of composite filamentsdisposed around the diffusion barrier and separated from the wirestabilizing matrix by the diffusion barrier. The wire stabilizing matrixincludes, consists essentially of, or consists of Cu. The diffusionbarrier includes, consists essentially of, or consists of a Ta—W alloy(e.g., a Ta alloy containing 0.2%-10% W). One or more, or even each, ofthe composite filaments includes, consists essentially of, or consistsof (i) a plurality of monofilaments, and (ii) (iii) a claddingsurrounding the plurality of monofilaments. The composite-filamentcladding includes, consists essentially of, or consists of Cu. Thediffusion barrier extends through an axial dimension of the wire.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The core of one or more, or even each,of the monofilaments may include, consist essentially of, or consist ofan alloy, pseudo-alloy, or mixture containing Nb and one or more of Ti,Zr, Hf, Ta, Y, or La (e.g., Nb—Ti). The core of one or more, or eveneach, of the monofilaments may include, consist essentially of, orconsist of Nb₃Sn. The diffusion barrier may include, consist essentiallyof, or consist of Ta-3W. The diffusion barrier may additionally containone or more alloying elements selected from the group consisting of Ru,Pt, Pd, Rh, Os, Ir, Mo, Re, or Si. The cross-sectional thickness and/orcross-sectional area of the diffusion barrier may be substantiallyconstant along the thickness of the wire. One or more, or even each, ofthe composite filaments may have a hexagonal cross-sectional shape(i.e., in cross-section perpendicular to the axial dimension of thewire). One or more, or even each, of the monofilaments may have ahexagonal cross-sectional shape (i.e., in cross-section perpendicular tothe axial dimension of the wire).

The wire may include a stabilizing element disposed within the pluralityof composite filaments or within or proximate the inner wire stabilizingmatrix. The stabilizing element may include, consist essentially of, orconsist of a Ta alloy containing 0.2%-10% W. At least a portion of thestabilizing element may be located substantially at the central core ofthe superconducting wire. The stabilizing element may occupy less thanapproximately 20% of a cross-section of the wire, less thanapproximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The stabilizing elementmay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire.

In another aspect, embodiments of the invention feature asuperconducting wire possessing diffusion resistance and mechanicalstrength. The superconducting wire includes, consists essentially of, orconsists of an outer wire matrix, a diffusion barrier disposed withinthe wire matrix, and a plurality of composite filaments surrounded bythe diffusion barrier and separated from the outer wire matrix by thediffusion barrier. The outer wire matrix includes, consists essentiallyof, or consists of Cu. The diffusion barrier includes, consistsessentially of, or consists of a Ta—W alloy (e.g., a Ta alloy containing0.2%-10% W). One or more, or even each, of the composite filamentsincludes, consists essentially of, or consists of (i) a plurality ofmonofilaments and (ii) a cladding surrounding the plurality ofmonofilaments. The composite-filament cladding may include, consistessentially of, or consist of Cu. One or more, or even each, of themonofilaments includes, consists essentially of, or consists of a coreand a cladding surrounding the core. The monofilament core may include,consist essentially of, or consist of Nb. The monofilament cladding mayinclude, consist essentially of, or consist of Cu. The diffusion barrierextends through an axial dimension of the superconducting wire. Thediffusion barrier occupies 1%-20%, 2%-15%, or 3%-10% of thecross-sectional area of the superconducting wire.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The core of one or more, or even each,of the monofilaments may include, consist essentially of, or consist ofan alloy, pseudo-alloy, or mixture containing Nb and one or more of Ti,Zr, Hf, Ta, Y, or La (e.g., Nb—Ti). The core of one or more, or eveneach, of the monofilaments may include, consist essentially of, orconsist of Nb₃Sn. The diffusion barrier may include, consist essentiallyof, or consist of Ta-3W. The diffusion barrier may additionally containone or more alloying elements selected from the group consisting of Ru,Pt, Pd, Rh, Os, Ir, Mo, Re, or Si. The cross-sectional thickness and/orcross-sectional area of the diffusion barrier may be substantiallyconstant along the thickness of the wire. One or more, or even each, ofthe composite filaments may have a hexagonal cross-sectional shape(i.e., in cross-section perpendicular to the axial dimension of thewire). One or more, or even each, of the monofilaments may have ahexagonal cross-sectional shape (i.e., in cross-section perpendicular tothe axial dimension of the wire). The yield strength of thesuperconducting wire may be at least 75 MPa, or even at least 100 MPa.

The wire may include a stabilizing element disposed within the pluralityof composite filaments and surrounded by the diffusion barrier. Thestabilizing element may include, consist essentially of, or consist of aTa alloy containing 0.2%-10% W. At least a portion of the stabilizingelement may be located substantially at the central core of thesuperconducting wire. The stabilizing element may occupy less thanapproximately 20% of a cross-section of the wire, less thanapproximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The stabilizing elementmay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire.

In another aspect, embodiments of the invention feature asuperconducting wire possessing diffusion resistance and mechanicalstrength. The superconducting wire includes, consists essentially of, orconsists of a wire matrix and a plurality of composite filamentsembedded within the wire matrix. The wire matrix includes, consistsessentially of, or consists of Cu. One or more, or even each, of thecomposite filaments includes, consists essentially of, or consists of(i) a plurality of monofilaments, (ii) a diffusion barrier extendingthrough an axial dimension of the composite filament and surrounding theplurality of monofilaments, and (iii) a cladding surrounding thediffusion barrier, the diffusion barrier separating the cladding fromthe plurality of monofilaments. The composite-filament diffusion barrierincludes, consists essentially of, or consists of a Ta—W alloy (e.g., aTa alloy containing 0.2%-10% W). The composite-filament claddingincludes, consists essentially of, or consists of Cu. One or more, oreven each, of the monofilaments includes, consists essentially of, orconsists of a core and a cladding surrounding the core. The monofilamentcore may include, consist essentially of, or consist of Nb. Themonofilament cladding may include, consist essentially of, or consist ofCu. The diffusion barriers collectively occupy 1%-20%, 2%-15%, or 3%-10%of the cross-sectional area of the superconducting wire.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The core of one or more, or even each,of the monofilaments may include, consist essentially of, or consist ofan alloy, pseudo-alloy, or mixture containing Nb and one or more of Ti,Zr, Hf, Ta, Y, or La (e.g., Nb—Ti). The core of one or more, or eveneach, of the monofilaments may include, consist essentially of, orconsist of Nb₃Sn. The diffusion barrier may include, consist essentiallyof, or consist of Ta-3W. The diffusion barrier may additionally containone or more alloying elements selected from the group consisting of Ru,Pt, Pd, Rh, Os, Ir, Mo, Re, or Si. The cross-sectional thickness and/orcross-sectional area of the diffusion barrier may be substantiallyconstant along the thickness of the wire. One or more, or even each, ofthe composite filaments may have a hexagonal cross-sectional shape(i.e., in cross-section perpendicular to the axial dimension of thewire). One or more, or even each, of the monofilaments may have ahexagonal cross-sectional shape (i.e., in cross-section perpendicular tothe axial dimension of the wire). The yield strength of thesuperconducting wire may be at least 75 MPa, or even at least 100 MPa.

The wire may include a stabilizing element disposed within the pluralityof composite filaments. The stabilizing element may include, consistessentially of, or consist of a Ta alloy containing 0.2%-10% W. At leasta portion of the stabilizing element may be located substantially at thecentral core of the superconducting wire. The stabilizing element mayoccupy less than approximately 20% of a cross-section of the wire, lessthan approximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The stabilizing elementmay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire.

In yet another aspect, embodiments of the invention feature asuperconducting wire possessing diffusion resistance and mechanicalstrength. The superconducting wire includes, consists essentially of, orconsists of an inner wire stabilizing matrix, a diffusion barrierdisposed around the wire stabilizing matrix, and a plurality ofcomposite filaments disposed around the diffusion barrier and separatedfrom the wire stabilizing matrix by the diffusion barrier. The wirestabilizing matrix includes, consists essentially of, or consists of Cu.The diffusion barrier includes, consists essentially of, or consists ofa Ta—W alloy (e.g., a Ta alloy containing 0.2%-10% W). One or more, oreven each, of the composite filaments includes, consists essentially of,or consists of (i) a plurality of monofilaments, and (ii) (iii) acladding surrounding the plurality of monofilaments. Thecomposite-filament cladding includes, consists essentially of, orconsists of Cu. The diffusion barrier extends through an axial dimensionof the wire. The diffusion barrier occupies 1%-20%, 2%-15%, or 3%-10% ofthe cross-sectional area of the superconducting wire.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The core of one or more, or even each,of the monofilaments may include, consist essentially of, or consist ofan alloy, pseudo-alloy, or mixture containing Nb and one or more of Ti,Zr, Hf, Ta, Y, or La (e.g., Nb—Ti). The core of one or more, or eveneach, of the monofilaments may include, consist essentially of, orconsist of Nb₃Sn. The diffusion barrier may include, consist essentiallyof, or consist of Ta-3W. The diffusion barrier may additionally containone or more alloying elements selected from the group consisting of Ru,Pt, Pd, Rh, Os, Ir, Mo, Re, or Si. The cross-sectional thickness and/orcross-sectional area of the diffusion barrier may be substantiallyconstant along the thickness of the wire. One or more, or even each, ofthe composite filaments may have a hexagonal cross-sectional shape(i.e., in cross-section perpendicular to the axial dimension of thewire). One or more, or even each, of the monofilaments may have ahexagonal cross-sectional shape (i.e., in cross-section perpendicular tothe axial dimension of the wire). The yield strength of thesuperconducting wire may be at least 75 MPa, or even at least 100 MPa.

The wire may include a stabilizing element disposed within the pluralityof composite filaments or within or proximate the inner wire stabilizingmatrix. The stabilizing element may include, consist essentially of, orconsist of a Ta alloy containing 0.2%-10% W. At least a portion of thestabilizing element may be located substantially at the central core ofthe superconducting wire. The stabilizing element may occupy less thanapproximately 20% of a cross-section of the wire, less thanapproximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The stabilizing elementmay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately” and “substantially” mean±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. For example, a structure consistingessentially of multiple metals will generally include only those metalsand only unintentional impurities (which may be metallic ornon-metallic) that may be detectable via chemical analysis but do notcontribute to function. As used herein, “consisting essentially of atleast one metal” refers to a metal or a mixture of two or more metalsbut not compounds between a metal and a non-metallic element or chemicalspecies such as oxygen, silicon, or nitrogen (e.g., metal nitrides,metal silicides, or metal oxides); such non-metallic elements orchemical species may be present, collectively or individually, in traceamounts, e.g., as impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a schematic cross-sectional view of a tube utilized to form amonofilament in accordance with various embodiments of the invention;

FIG. 1B is a schematic cross-sectional view of a rod utilized to form amonofilament in accordance with various embodiments of the invention;

FIG. 1C is a schematic cross-sectional view of a monofilament utilizedto form a composite filament in accordance with various embodiments ofthe invention;

FIG. 2A is a schematic cross-sectional view of a tube utilized to form acomposite filament in accordance with various embodiments of theinvention;

FIG. 2B is a schematic cross-sectional view of a tube utilized to form adiffusion barrier within a composite filament in accordance with variousembodiments of the invention;

FIG. 2C is a schematic cross-sectional view of a stack of monofilamentsutilized to form a composite filament in accordance with variousembodiments of the invention;

FIG. 2D is a schematic cross-sectional view of a composite filament atan initial stage of fabrication in accordance with various embodimentsof the invention;

FIG. 2E is a schematic cross-sectional view of a composite filamentutilized to form superconducting wires in accordance with variousembodiments of the invention;

FIG. 3A is a schematic cross-sectional view of a tube utilized to form astabilizing element in accordance with various embodiments of theinvention;

FIG. 3B is a schematic cross-sectional view of a rod utilized to form astabilizing element in accordance with various embodiments of theinvention;

FIG. 3C is a schematic cross-sectional view of a stabilizing elementutilized to form stabilized composite filaments and/or superconductingwires in accordance with various embodiments of the invention;

FIG. 3D is a schematic cross-sectional view of a composite filamentincorporating a stabilizing element in accordance with variousembodiments of the invention;

FIG. 4A is a schematic cross-sectional view of a tube utilized to form asuperconducting wire in accordance with various embodiments of theinvention;

FIG. 4B is a schematic cross-sectional view of a stack of compositefilaments utilized to form a superconducting wire in accordance withvarious embodiments of the invention;

FIG. 4C is a schematic cross-sectional view of a tube utilized to form adiffusion barrier within a superconducting wire in accordance withvarious embodiments of the invention;

FIG. 4D is a schematic cross-sectional view of a superconducting wire atan initial stage of fabrication in accordance with various embodimentsof the invention;

FIG. 4E is a schematic cross-sectional view of a superconducting wire inaccordance with various embodiments of the invention;

FIG. 4F is a schematic cross-sectional view of a stabilizedsuperconducting wire at an initial stage of fabrication in accordancewith various embodiments of the invention;

FIG. 4G is a schematic cross-sectional view of a stabilizedsuperconducting wire in accordance with various embodiments of theinvention;

FIG. 5 is a cross-sectional micrograph of a superconducting wirefeaturing a Cu inner stabilizer and a diffusion barrier disposed aroundthe stabilizer in accordance with various embodiments of the invention;

FIG. 6 is a cross-sectional micrograph of a superconducting wirefeaturing a Cu outer matrix and a diffusion barrier disposed between theouter matrix and the wire filaments in accordance with variousembodiments of the invention;

FIG. 7A is a cross-sectional micrograph of a superconducting wirefeaturing a Cu outer matrix and a Ta-3W diffusion barrier disposedbetween the outer matrix and the wire composite filaments in accordancewith various embodiments of the invention;

FIG. 7B is a cross-sectional micrograph of a superconducting wirefeaturing a Cu matrix and a Ta-3W diffusion barrier surrounding eachinternal composite filament in accordance with various embodiments ofthe invention;

FIG. 7C is a cross-sectional micrograph of a superconducting wirefeaturing a Cu matrix and composite filaments therein;

FIG. 7D is a cross-sectional micrograph of a superconducting wirefeaturing a Cu matrix, composite filaments therein, and a Ta-3W innerstabilizing core in accordance with various embodiments of theinvention;

FIG. 8A is a graph of yield strength and ultimate tensile strength ofthe wire depicted in FIG. 7C after two different annealing treatments;

FIG. 8B is a graph of yield strength and ultimate tensile strength ofthe wire depicted in FIG. 7D after annealing;

FIG. 8C is a graph of yield strengths and ultimate tensile strengths ofthe wires depicted in FIGS. 7A and 7B after annealing;

FIG. 9A is a cross-sectional micrograph of a superconducting wirefeaturing inner Sn-containing filaments, outer Nb-containing compositefilaments, and a Ta-3W diffusion barrier surrounding the filaments andseparating them from an outer Cu stabilizing matrix;

FIG. 9B is a cross-sectional micrograph of the wire depicted in FIG. 9Aafter annealing at 210° for 72 hours;

FIG. 9C is a cross-sectional micrograph of the wire depicted in FIG. 9Aafter annealing at 210° for 72 hours, 400° C. for 48 hours, and 640° C.for 48 hours; and

FIG. 9D is a graph of electrical resistance as a function of temperaturefor the wires depicted in FIGS. 9B and 9C.

DETAILED DESCRIPTION

FIGS. 1A-1C depict components of an exemplary monofilament 100 andconstituent components thereof. In accordance with embodiments of theinvention, a rod 105 is disposed within a tube 110 that includes,consists essentially of, or consists of Cu or a Cu alloy (e.g., bronze).The composition of the rod 105 may be selected based on the particularmetallic superconductor desired in the final wire. For example, the rod105 may include, consist essentially of, or consist of Nb, Ti, Nb—Ti, oran alloy thereof. In other examples, the rod 105 may include, consistessentially of, or consist of Nb alloyed with one or more of Ti, Zr, Hf,Ta, Y, or La. Such alloying elements may be individually or collectivelypresent within the rod 105 (and thus within the core of a monofilament100) is concentrations of, for example, 0.2%-10% (e.g., 0.2%-5%, or0.5%-1%). The rod 105 clad with the tube 110 may subsequently be drawndown to reduce its diameter to, for example, between 0.5 inch and 1.5inches. The clad rod may be drawn down in multiple stages and may beheat treated during and/or after any or each of the drawing steps for,e.g., strain relief. Once drawn down, the clad rod may be drawn througha shaped die in order to fabricate the monofilament 100 shaped forefficient stacking with other monofilaments. For example, as shown inFIG. 1C, a hexagonal die may be utilized to form a monofilament 100having a hexagonal cross-section. In other embodiments, monofilamentsmay have other cross-sections, e.g., square, rectangular, triangular,etc.

Once a monofilament 100 is fabricated, other monofilaments 100 may alsobe fabricated in the same manner, or one or more monofilaments 100 maybe divided into multiple pieces. Multiple monofilaments may be stackedtogether to form at least a portion of a composite filament. FIGS. 2A-2Edepict various components and assembly of a composite filament 200. Asshown in FIG. 2C, multiple monofilaments 100 may be stacked together inan arrangement that will subsequently become at least a portion of thecore of composite filament 200. While FIG. 2C depicts the stacking of 19different monofilaments 100, embodiments of the invention may includemore or fewer monofilaments 100. The stacked assembly of monofilaments100 may be disposed within a tube 205 that includes, consistsessentially of, or consists of Cu or a Cu alloy (e.g., bronze). As shownin FIG. 2B, a tube 210 may be disposed within the tube 205 and aroundthe stack of monofilaments 100; this tube 210 will become the diffusionbarrier 215 in the final composite filament and retard or substantiallyprevent interdiffusion between the monofilaments 100 and the material oftube 205, which becomes the outer matrix 220 of the resulting compositefilament. Thus, the tube 210 may include, consist essentially of, orconsist of a Ta alloy such as Ta—W (e.g., Ta-3W). Before and/or afterthe monofilaments 100 are disposed within the tube 205 and the tube 210,the monofilaments 100, the tube 205, and/or the tube 210 may be cleanedand/or etched (e.g., via a cleaning agent including, consistingessentially of, or consisting of one or more acids) to, for example,remove surface oxides and/or other contaminants.

The tube 210 may be fabricated via alloying of pure Ta with the one ormore alloying elements disposed within the diffusion barrier. Forexample, for diffusion barriers (and thus tubes 210) including,consisting essentially of, or consisting of an alloy of Ta and W, Ta andW may be alloyed together in the desired amounts via a process such aselectron-beam melting and/or arc melting. The resulting material may befabricated into a sheet, and the sheet may be formed into a tube by,e.g., rolling, deep drawing, extrusion, pilgering, etc.

As shown in FIG. 2D, the tube 205 and tube 210 may be compacted onto themonofilaments 100 by, e.g., swaging, extruding, and/or rolling. The cladstacked monofilaments 100 may be annealed to promote bonding between thevarious monofilaments 100 in the stacked assembly. For example, the cladstacked monofilaments may be annealed at a temperature betweenapproximately 300° C. and approximately 500° C. (e.g., approximately400° C.) for a time of approximately 0.5 hour and approximately 3 hours(e.g., approximately 1 hour). Advantageously, the presence of thediffusion barrier 215 between the monofilaments 100 and the outer matrix220 substantially prevents diffusion between the Cu of the matrix 220and the monofilaments 100, thereby preventing the formation of metallicphases having low electrical conductivity (e.g., electrical conductivitylower than Cu and/or than the material of matrix 220). The diffusionbarrier 215 also provides additional mechanical strength to the finalwire, given its superior mechanical properties (e.g., strength, yieldstrength, tensile strength, stiffness, Young's modulus, etc.) comparedwith those of the outer matrix 220 and/or the monofilaments 100,particularly after the extended high-temperature heat treatmentsutilized for reactive formation of the superconducting phase in thewire.

The resulting assembly may be drawn down one or more times to reduce itsdiameter, and may subsequently be drawn through a shaped die in order toprovide composite filament 200 with a cross-sectional shape configuredfor efficient stacking. For example, as shown in FIG. 2E, a hexagonaldie may be utilized to form a composite filament 200 having a hexagonalcross-section. In other embodiments, composite filaments 200 may haveother cross-sections, e.g., square, rectangular, triangular, round,off-round, elliptical, etc. In various embodiments, the cross-sectionalsize and/or shape of the composite filament 200 after processing andshaping is equal to the cross-sectional size and/or shape of themonofilament 100 utilized in the initial stacked assembly before beingreduced in size (i.e., shown in FIG. 2C). (Although the diffusionbarrier 215 resulting from the incorporation of tube 210 is depicted inFIGS. 2D and 2E as having a variable cross-sectional thickness, invarious embodiments of the invention the diffusion barrier 215 has asubstantially uniform cross-sectional thickness around itscircumference, and diffusion barrier 215 may have the form, incross-section, of an annular ring (for example, a ring disposed tightlyaround the filaments (or other structures) therewithin), as shown inFIGS. 5 and 6; diffusion barriers having an annular cross-section inaccordance with embodiments of the invention generally have the form oftubes that extend along the axial dimension of the wire.)

Superconducting wires in accordance with embodiments of the inventionmay also incorporate stabilizing elements that provide even moremechanical strength while not compromising the drawability and/orelectrical performance of the wires. FIGS. 3A-3C depict the fabricationof a stabilizing element 300 via a method similar to that detailed abovefor monofilaments 100. In accordance with embodiments of the invention,a rod 305 is disposed within a tube 310 that includes, consistsessentially of, or consists of Cu or a Cu alloy. The rod 305 mayinclude, consist essentially of, or consist of one or more metals havingmechanical strength (e.g., tensile strength, yield strength, etc.)greater than that of rods 105 utilized to fabricate monofilaments 100.For example, the rod 305 may include, consist essentially of, or consistof Ta or a Ta alloy (e.g., a Ta—W alloy such as Ta-3W) or any othermaterial disclosed herein as suitable for diffusion barriers. In otherembodiments, the rod 305 may include, consist essentially of, or consistof a Nb alloy having greater mechanical strength than substantially pureNb. For example, rods 305 (and therefore stabilizing elements) inaccordance with embodiments of the invention may include, consistessentially of, or consist of an alloy of Nb with one or more of Hf, Ti,Zr, Ta, V, Y, Mo, or W. For example, stabilizing elements in accordancewith embodiments of the invention may include, consist essentially of,or consist of Nb C103 alloy, which includes approximately 10% Hf,approximately 0.7%-1.3% Ti, approximately 0.7% Zr, approximately 0.5%Ta, approximately 0.5% W, and the balance Nb. In other embodiments,stabilizing elements may include, consist essentially of, or consist ofa Nb B66 alloy and/or a Nb B77 alloy.

The rod 305 clad with the tube 310 may subsequently be drawn down toreduce its diameter to, for example, between 0.5 inch and 1.5 inches.The clad rod may be drawn down in multiple stages and may be heattreated during and/or after any or each of the drawing steps for, e.g.,strain relief. Once drawn down, the clad rod may be drawn through ashaped die in order to fabricate the stabilizing element 300 shaped forefficient stacking with monofilaments 100 and/or composite filaments200. For example, as shown in FIG. 3C, a hexagonal die may be utilizedto form a stabilizing element 300 having a hexagonal cross-section. Inother embodiments, stabilizing elements 300 may have othercross-sections, e.g., square, rectangular, triangular, etc. In variousembodiments, stabilizing elements 300 may have cross-sectional sizesand/or shapes substantially the same as cross-sectional sizes and/orshapes of monofilaments 100 and/or composite filaments 200.

Once fabricated, one or more stabilizing elements 300 may be insertedinto a stack of monofilaments 100, and the resulting assembly may besurrounded with a diffusion-barrier material and a matrix material,drawn down, and optionally shaped to form a stabilized compositefilament 315 (e.g., as described above with reference to FIGS. 2A-2E)that incorporates a diffusion barrier 215 between the monofilaments 100and stabilizing element(s) 300 and the outer matrix 220, as shown inFIG. 3D. In various embodiments of the invention, composite filament mayinclude a diffusion barrier between the stabilizing element 300 and theremaining monofilaments 100 in order to retard or substantially preventinterdiffusion therebetween. In various embodiments, the stabilizingelement 300 may be replaced or supplemented with an internal stabilizingmatrix that includes, consists essentially of, or consists of, e.g., Cuor a Cu alloy, and such regions may be separated from monofilaments 100via one or more diffusion barriers.

In embodiments of the invention incorporating stabilizing elements aswell as diffusion barriers, the amount of cross-sectional area of thewire imparting additional mechanical strength may be beneficiallydivided between the diffusion barrier(s) and the stabilizing element(s).That is, the more cross-sectional area of the wire occupied by one ormore stabilizing elements, the less cross-sectional area of the wireneed be occupied by the diffusion barrier(s), as long as each diffusionbarrier has sufficient thickness to retard or substantially eliminatediffusion between the various portions of the wire. Conversely, the useof diffusion barriers in accordance with embodiments of the inventionenables the use of one or more stabilizing elements that themselvescollectively occupy less of the cross-sectional area of the wire whilestill imparting the desired mechanical strength (and/or other mechanicalproperties) to the wire. In various embodiments, the diffusionbarrier(s) may collectively occupy at least 2%, at least 3%, at least4%, or at least 5% of the cross-sectional area of the wire. In variousembodiments, the diffusion barrier(s) may collectively occupy less than15%, less than 12%, less than 10%, less than 9%, or less than 8% of thecross-sectional area of the wire. In embodiments of the inventionfeaturing stabilizing elements, the stabilizing elements and diffusionbarriers may collectively occupy less than 25%, less than 20%, or lessthan 15% of the cross-sectional area of the wire. Stabilizing elementsthemselves may occupy less than 15% or less than 10% (e.g.,approximately 2% to approximately 8%, or approximately 5% toapproximately 15%) of the cross-sectional area of the wire. Stabilizingelements may occupy at least 2%, at least 3%, at least 5%, or at least8% of the cross-sectional area of the wire.

In addition to or instead of being incorporated within one or morecomposite filaments 200, 315, diffusion barriers in accordance withembodiments of the present invention may be disposed between an outerstabilizing matrix (and/or an inner stabilizing matrix and/or stabilizerproximate the center of the wire) and the composite filaments toadvantageously retard or substantially prevent interdiffusion within thesuperconducting wire. That is, superconducting wires and/or wirepreforms may be fabricated utilizing diffusion barriers disposed aroundassemblies of composite filaments 200, stabilized composite filaments315, and/or composite filaments lacking their own diffusion barriers.FIGS. 4A-4E depict various stages of the fabrication of an exemplarysuperconducting wire 400. As shown in FIG. 4B, multiple compositefilaments 405 each lacking their own internal diffusion barriers may bestacked together in an arrangement that will subsequently become atleast a portion of the core of superconducting wire 400. Each compositefilament 405 may be fabricated, for example, similarly to compositefilament 200 detailed above but without incorporation of the diffusionbarrier 215 arising from the use of tube 210 during fabrication. Inother embodiments, the stack of composite filaments may include or becomposed of composite filaments 200, composite filaments 315, and/ormixtures thereof with or without composite filaments 405. While FIG. 4Bdepicts the stacking of 18 different composite filaments 405,embodiments of the invention may include more or fewer compositefilaments.

The stacked assembly of composite filaments may be disposed within atube 410 that includes, consists essentially of, or consists of Cu or aCu alloy. In addition, as shown in FIG. 4C, a tube 210 may be disposedaround the stacked assembly of composite filaments and within the tube410 and may therefore form a diffusion barrier in the final wire. Beforeand/or after the composite filaments are disposed within the tube 510and the tube 210, the composite filaments, the tube 210, and/or the tube410 may be cleaned and/or etched (e.g., via a cleaning agent including,consisting essentially of, or consisting of one or more acids) to, forexample, remove surface oxides and/or other contaminants. As shown inFIG. 4D, the tube 410 and the tube 210 may be compacted onto thecomposite filaments by, e.g., swaging, extruding, and/or rolling, andtube 210 may become diffusion barrier 415, and tube 410 may become outermatrix 420. The clad stacked composite filaments may be annealed topromote bonding between the various composite filaments in the stackedassembly. For example, the clad stack may be annealed at a temperaturebetween approximately 300° C. and approximately 500° C. (e.g.,approximately 400° C.) for a time of approximately 0.5 hour andapproximately 3 hours (e.g., approximately 1 hour). Advantageously, thepresence of the diffusion barrier 415 between the composite filaments405 and the outer matrix 420 substantially prevents diffusion betweenthe Cu of the matrix 420 and the composite filaments 405, therebypreventing the formation of metallic phases having low electricalconductivity (e.g., electrical conductivity lower than Cu and/or thanthe material of matrix 420). The resulting assembly may be drawn downone or more times to reduce its diameter, as shown in FIG. 4E. Before orafter drawing, the superconducting wire 400 may be annealed to, e.g.,relax residual stresses and/or promote recrystallization therein.

As shown in FIGS. 4F and 4G, a similar methodology may be utilized tofabricate stabilized semiconducting wires 425 that incorporate one ormore diffusion barriers 415 as well as one or more stabilizing elements300. For example, the assembly of stacked composite filaments may definetherewithin one or more voids each sized and shaped to accommodate oneor more stabilizing elements 300. Before or after the compositefilaments are disposed within the tube 410 and the tube 210, one or morestabilizing elements 300 may be disposed within each of the voids, asshown in FIG. 4F. The resulting assembly may have its diameter reducedby, e.g., drawing and/or extrusion, as shown in FIG. 4G. In variousembodiments, a diffusion barrier may be disposed between stabilizingelement(s) 300 and the filaments within the wire or wire preform,particularly in embodiments in which the stabilizing element includes,consists essentially of, or consists of Cu. For example, a tube of thedesired diffusion barrier material may be disposed around thestabilizing element when the wire preform assembly is assembled, and theentire assembly may be drawn down to the desired wire dimensions. WhileFIGS. 4F and 4G depicts the superconducting wire 425 having a singlestabilizing element 300 disposed substantially at the center of thestacked assembly of composite filaments, in accordance with embodimentsof the invention, one or more stabilizing elements 300 may be disposedat other locations within the stacked assembly in addition to or insteadof the stabilizing element 300 disposed at the center.

In various embodiments, the superconducting wire 400, 425 lacks adiffusion barrier 415 therewithin, and thus, tube 210 is not utilized information thereof, and diffusion barriers 215 in one or more of theindividual composite filaments are utilized to retard or substantiallyprevent interdiffusion. In other embodiments, as shown in FIG. 4D-4G,the individual composite filaments 405 may lack diffusion barrierstherewithin, and diffusion barrier 415 is present within thesuperconducting wire 400, 425. In such embodiments, the tubes 110 and/or205 may incorporate therewithin Sn which advantageously reacts with theNb of the filaments during subsequent thermal processing to form asuperconducting phase (e.g., Nb₃Sn). In other embodiments, diffusionbarrier 415 is present in addition to diffusion barriers 215 within theindividual composite filaments.

In various embodiments, the superconducting wire 400, superconductingwire 425, composite filament 4015, composite filament 200, and/orstabilized composite filament 315 may be mechanically processed fordiameter reduction and/or to promote bonding between their constituentelements prior to wire drawing steps. For example, the superconductingwire 400, superconducting wire 425, composite filament 4015, compositefilament 200, and/or stabilized composite filament 315 may be extruded,swaged, and/or rolled prior to the final drawing step(s). In variousembodiments, the superconducting wire 400, superconducting wire 425,composite filament 4015, composite filament 200, and/or stabilizedcomposite filament 315 may be heat treated during and/or after each ofmultiple different drawing steps for strain relief. For example, duringand/or after one or more of the drawing steps, the superconducting wire400, superconducting wire 425, composite filament 4015, compositefilament 200, and/or stabilized composite filament 315 may be annealedat temperatures from approximately 360° C. to approximately 420° C. fora time period of, e.g., approximately 20 hours to approximately 40hours.

In various embodiments of the present invention, the superconductingwire 400 or superconducting wire 425 may be cooled below the criticaltemperature of the filaments therewithin and utilized to conductelectrical current. In some embodiments, multiple superconducting wires400 and/or superconducting wires 425 are coiled together to form asingle superconducting cable.

While some superconducting wires 400, 425 (e.g., those incorporatingNb—Ti-containing filaments) may be utilized directly in superconductingapplications, the fabrication processes for various othersuperconducting wires 400, 425 may incorporate one or more steps toincorporate a portion of the superconducting phase. For example, Nb₃Snsuperconducting phases, once formed, are typically brittle and may notbe further drawn or otherwise mechanically deformed without damage.Thus, embodiments of the present invention may be utilized to fabricatesuperconducting wires 400, 425 that incorporate Nb and Sn separate fromeach other; once the wires 400, 425 are mostly or fully fabricated, thewires 400, 425 may be annealed to interdiffuse the Nb and Sn and formthe superconducting Nb₃Sn phase therewithin. For example, the drawn wiremay be annealed at temperatures from approximately 600° C. toapproximately 700° C. for a time period of, e.g., approximately 30 hoursto approximately 200 hours. In various embodiments, one or more of theCu-based tubes 110, 205, or 310 may incorporate Sn therewithin; e.g.,one or more of the tubes may include, consist essentially of, or consistof a Cu—Sn alloy (that includes, e.g., 13-15% Sn). Such materials areductile, enabling the fabrication of the various filaments and wires asdetailed herein. Thereafter, the wire 400, 425 may be annealed,resulting in interdiffusion and the formation of the superconductingNb₃Sn phase at least at the interface between the Nb and the Cu—Sn.

In other embodiments, pure Sn or an Sn alloy (e.g., an Sn alloy with Cuor magnesium (Mg)) may be incorporated (e.g., in the form of a rod ortube) within one or more of the stacks utilized to form compositefilaments 200, stabilized composite filaments 315, and/or wires 400,425; after formation of the composite filaments 200, stabilizedcomposite filaments 315, and/or wires 400, 425 as detailed herein, anannealing step may be performed to form the superconducting Nb₃Sn phase.

FIG. 5 is a cross-sectional view of a superconducting wire 500incorporating a diffusion barrier in accordance with embodiments of thepresent invention. As shown, the diffusion barrier 510 is disposedbetween a Cu stabilizing core 520 of the wire 500 and an outer bronzematrix 530 containing Nb-based filaments 540. FIG. 6 is across-sectional view of another superconducting wire 600 incorporating adiffusion barrier in accordance with embodiments of the presentinvention. As shown, the diffusion barrier 610 is disposed between innerSb—Cu—Nb-based filaments 620 at the core of the wire 600 and an outer Custabilizer 630.

EXAMPLES

A series of experiments were performed to evaluate Ta-3W diffusionbarriers in terms of processability and performance (i.e., retardingdiffusion within the final wire). The fabrication of the initialNb-based filaments began by cladding annealed Nb rods with oxygen-freeelectronic (OFE) Cu tubing by sinking a tube having a slightly largerinner diameter than the diameter of the rod. (As known in the art, OFECu is at least 99.99% pure and has an oxygen content no greater than0.0005%.) The Cu to Nb ratio in each of the monofilaments wasapproximately 1:3. The clad rods were drawn down to 3.66 mm round andthen pulled through a 3.05 mm flat-to-flat hexagonal die. The resultinghexagonal monofilaments were stacked in groups of 19, placed intoanother Cu tube, and cold drawn to 3.66 mm to form composite filaments,which were subsequently pulled through a 3.05 mm flat-to-flat hexagonaldie.

In order to form a first experimental sample having a single diffusionbarrier, 19 of the composite filaments where stacked together and placedwithin a seam-welded Ta-3W tube having a 12.7 mm diameter and a 0.38 mmwall thickness. This assembly was placed within a Cu stabilizer tubehaving a 16.5 mm diameter and a 1.5 mm wall thickness. The resultingassembly was cold drawn to 0.72 mm diameter, and a cross-sectionalmicrograph of the resulting wire 700 is shown in FIG. 7A. Portions ofwire 700 were drawn through a 3.05 mm flat-to-flat hexagonal die andstacked together as an 18-element assembly within a Cu tube having an18.3 mm diameter and a 1.5 mm wall thickness. The resulting assembly wascold drawn down to 0.72 mm diameter, and a cross-sectional micrograph ofthe resulting wire 710, in which each “bundle” of composite filaments issurrounded by its own diffusion barrier, is shown in FIG. 7B. Wires 700,710 were fabricated to evaluate the suitability of the Ta-3W diffusionbarrier to be co-processed with the Nb and Cu elements of the wire, andthus no Sn elements were incorporated within the wires. As shown inFIGS. 7A and 7B, the diffusion barriers remained continuous andgenerally conformed to the shape of the filament stacks they surrounded,despite being subjected to an area reduction of over 500:1. Note thatthe thickness of wire 700 is slightly non-uniform in the locationcorresponding to the seam weld of the initial Ta-3W tube; suchnon-uniformities may be addressed via the use of seamless tubes in theoriginal fabrication or by optimizing the weld.

For comparative purposes, two additional wires similar to wire 710 butlacking the diffusion barrier were fabricated. FIG. 7C is across-sectional micrograph of a wire 720 having a Cu stabilizer core butlacking the diffusion barrier around each of the 19-element compositefilament bundles. FIG. 7D is a cross-sectional micrograph of a wire 730identical to wire 720 but featuring an inner core stabilizer composed ofTa-3W rather than Cu. The inner core stabilizer of wire 730 occupiesapproximately 2.5% of the cross-sectional area of wire 730.

The mechanical properties of portions of the various wires 700, 710,720, 730 taken at different wire diameters were tested after an annealdesigned to mimic an example heat treatment during superconducting wirefabrication. The various wires were annealed at 3 hours for 700° C., andthe resulting yield strengths and ultimate tensile strengths of thewires were measured in accordance with ASTM E8/E8M-15a, Standard TestMethods for Tension Testing of Metallic Materials, ASTM International,West Conshohocken, Pa., 2015, the entire disclosure of which isincorporated by reference herein. Additional portions of the wire 720were annealed at 250° C. for 8 hours for comparative purposes. FIG. 8Ais a graph of yield strength (“yield”) and ultimate tensile strength(“UTS”) for samples of wire 720 annealed at either 250° C. for 8 hoursor 700° C. for 3 hours. As shown, the wire 720 softens considerablyduring the higher-temperature anneal cycle. FIG. 8B is a similar graphof yield strength and UTS for samples of wire 730 annealed at 700° C.for 3 hours. As shown, the wire 730 maintains a higher level of strengthafter annealing when compared to wire 720 due to the presence of theTa-3W stabilizing core. FIG. 8C is a similar graph of yield strength andUTS for samples of wires 700 and 710 annealed at 700° C. for 3 hours. Asshown, both wires 700 and 710 maintain their strengths at levels evensuperior to that of wire 730 due to the presence of the Ta-3W diffusionbarriers therewithin. It is clear from these results that wiresincorporating both Ta-3W diffusion barriers and stabilizers will possessmechanical properties at least comparable to, if not superior to, thoseof wires 700, 710, and 730. Moreover, such wires would not only possesssuperior mechanical properties, but would also resist interdiffusiontherewithin in a fashion superior to wires possessing only stabilizers.Thus, wires in which the total cross-sectional area devoted todiffusion-barrier and stabilizing-element materials is dividedtherebetween will possess an advantageous combination of diffusionresistance and mechanical strength while still not compromising theelectrical performance of the wire (due to, e.g., an excessively largeamount of the wire cross-sectional area being occupied bynon-superconducting materials).

An additional experiment was performed to evaluate the efficacy of theTa-3W diffusion barrier in retarding diffusion within a superconductingwire. The wire 900 featured a 37-element stack surrounded by a Ta-3Wdiffusion barrier, which was in turn surrounded by an outer Cu wirematrix. The inner 7 elements of the wire were formed from Cu-clad Sn—Tirods, while the outer 30 elements of the wire were formed from Cu-cladNb composite filaments. The initial Ta-3W diffusion barrier tube was anoverlapped (rather than welded) tube having a diameter of 12.7 mm and awall thickness of 0.38 mm. FIG. 9A is an optical micrograph of the wire900 as drawn down to 0.72 mm diameter. The Ta-3W diffusion barrier isintact around the inner elements, and the thickness of the diffusionbarrier is slightly thicker in the region of the tube overlap. Thediffusion barrier thickness was approximately 0.02 mm, and the Nbfilaments have diameters of approximately 0.015 mm. This filamentthickness is considerably larger than appropriate for a commercialsuperconducting wire but was adequate for this demonstration of wirefabrication and diffusion resistance. FIG. 9B is an inverted-contrastscanning electron microscopy (SEM) micrograph of the wire 900 afterannealing at 210° C. for a period of 72 hours. This annealing cycle wasmeant to simply anneal the outer Cu matrix without implicating thediffusion barrier, as at these annealing conditions only short-rangesolid-state diffusion results.

FIG. 9C is a micrograph of wire 900 after an annealing cyclerepresentative of the process to fabricate a superconducting wire via aninternal diffusion and reaction process. The wire was first annealed at210° C. for a period of 72 hours (as in FIG. 9B), followed by anneals of400° C. for 48 hours and 640° C. for 48 hours. As shown, the innerSn-based filaments have lost definition as the Sn has begun to reactwith the Nb of the outer Nb-based filaments. In addition, the grain sizeof the outer Cu matrix has increased after the more aggressive anneal.Despite the more aggressive annealing cycle, the diffusion barrier isintact, and no diffusion into the outer Cu matrix was detected. FIG. 9Dis a graph of electrical resistance as a function of temperature for thewire 900 after only the first anneal cycle (line 910) and after the moreaggressive annealing sequence (line 920). As shown, after the moreaggressive annealing sequence the wire 900 has begun to exhibit asuperconducting transition at approximately 17.2 K. The residualresistivity ratio (RRR) of the wire 900 after (1) only the first annealand (2) after the more aggressive annealing sequence, comparingresistances at 300K and at 19K, was measured using a four-point probeand a current of 0.5 Amperes. The RRR of wire 900 after only the firstanneal was 113±3, and the RRR of wire 900 after the more aggressiveannealing sequence was 247±7. The superior RRR after the three-annealsequence is due at least in part to the diffusion resistance of thediffusion barrier preventing contamination of the Cu matrix.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A superconducting wire possessing diffusionresistance and mechanical strength, the superconducting wire comprising:an outer wire matrix comprising Cu; disposed within the wire matrix, adiffusion barrier comprising a Ta—W alloy containing 0.2%-10% W; and aplurality of composite filaments surrounded by the diffusion barrier andseparated from the outer wire matrix by the diffusion barrier, wherein:each composite filament comprises (i) a plurality of Nb-basedmonofilaments and (ii) a cladding comprising Cu surrounding theplurality of monofilaments, the diffusion barrier occupies 2%-15% of across-sectional area of the superconducting wire, and the diffusionbarrier extends through an axial dimension of the superconducting wire.2. The wire of claim 1, wherein each monofilament comprises a corecomprising Nb and, surrounding the core, a cladding comprising Cu. 3.The wire of claim 1, wherein at least one monofilament comprises Nb andat least one of Ti, Zr, Hf, Ta, Y, or La.
 4. The wire of claim 1,wherein at least one monofilament comprises Nb and Sn.
 5. The wire ofclaim 1, wherein the diffusion barrier comprises Ta-3W.
 6. The wire ofclaim 1, wherein at least one of the composite filaments has a hexagonalcross-sectional shape and/or at least one of the monofilaments has ahexagonal cross-sectional shape.
 7. The wire of claim 1, furthercomprising a stabilizing element extending through the axial dimensionof the superconducting wire, the stabilizing element comprising (i) Ta,(ii) a Ta alloy containing 0.2%-10% W, or (iii) an alloy of Nb and atleast one of Hf, Ti, Zr, Ta, V, Y, Mo, or W.
 8. The wire of claim 1,wherein a yield strength of the superconducting wire is at least 100MPa.
 9. A superconducting wire possessing diffusion resistance andmechanical strength, the superconducting wire comprising: an outer wirematrix comprising Cu; disposed within the wire matrix, a diffusionbarrier comprising a Ta—W alloy and one or more alloying elementsselected from the group consisting of Ru, Pt, Pd, Rh, Os, Ir, Mo, Re, orSi; and a plurality of composite filaments surrounded by the diffusionbarrier and separated from the outer wire matrix by the diffusionbarrier, wherein: each composite filament comprises (i) a plurality ofNb-based monofilaments and (ii) a cladding comprising Cu surrounding theplurality of monofilaments, the diffusion barrier occupies 2%-15% of across-sectional area of the superconducting wire, and the diffusionbarrier extends through an axial dimension of the superconducting wire.10. The wire of claim 9, wherein the Ta—W alloy contains 0.2%-10% W. 11.The wire of claim 9, wherein the Ta—W alloy is Ta-3W.
 12. The wire ofclaim 9, wherein each monofilament comprises a core comprising Nb and,surrounding the core, a cladding comprising Cu.
 13. The wire of claim 9,wherein at least one monofilament comprises Nb and at least one of Ti,Zr, Hf, Ta, Y, or La.
 14. The wire of claim 9, wherein at least onemonofilament comprises Nb and Sn.
 15. The wire of claim 9, furthercomprising a stabilizing element extending through the axial dimensionof the superconducting wire, the stabilizing element comprising (i) Ta,(ii) a Ta alloy containing 0.2%40% W, or (iii) an alloy of Nb and atleast one of Hf, Ti, Zr, Ta, V, Y, Mo, or W.
 16. A superconducting wirepossessing diffusion resistance and mechanical strength, thesuperconducting wire comprising: an inner wire stabilizing matrixcomprising Cu; disposed around the wire stabilizing matrix, a diffusionbarrier comprising a Ta—W alloy containing 0.2%-10% W; and a pluralityof composite filaments disposed around the diffusion barrier andseparated from the wire stabilizing matrix by the diffusion barrier,wherein: each composite filament comprises (i) a plurality of Nb-basedmonofilaments and (ii) a cladding comprising Cu surrounding theplurality of monofilaments, the diffusion barrier occupies 2%-15% of across-sectional area of the superconducting wire, and the diffusionbarrier extends through an axial dimension of the wire.
 17. The wire ofclaim 16, wherein each monofilament comprises a core comprising Nb and,surrounding the core, a cladding comprising Cu.
 18. The wire of claim16, wherein at least one monofilament comprises Nb and at least one ofTi, Zr, Hf, Ta, Y, or La.
 19. The wire of claim 16, wherein at least onemonofilament comprises Nb and Sn.
 20. The wire of claim 16, wherein thediffusion barrier comprises Ta-3W.
 21. The wire of claim 16, wherein thediffusion barrier additionally contains one or more alloying elementsselected from the group consisting of Ru, Pt, Pd, Rh, Os, Ir, Mo, Re, orSi.
 22. The wire of claim 16, wherein at least one of the compositefilaments has a hexagonal cross-sectional shape and/or at least one ofthe monofilaments has a hexagonal cross-sectional shape.
 23. The wire ofclaim 16, further comprising a stabilizing element extending through theaxial dimension of the superconducting wire, the stabilizing elementcomprising (i) Ta, (ii) a Ta alloy containing 0.2%40% W, or (iii) analloy of Nb and at least one of Hf, Ti, Zr, Ta, V, Y, Mo, or W.
 24. Thewire of claim 16, wherein a yield strength of the superconducting wireis at least 100 MPa.